Inkers for printing plates which have achieved commercial acceptance generally comprise from two to four form rollers which are positioned in rolling engagement with a printing plate. Each of the form rollers are usually in rolling engagement with one or more vibrator rollers to which ink is applied by a multitude of rollers in a train of rollers of varying diameters arranged in pyramid fashion. Ink is delivered to the train of rollers over a ductor roller which oscillates into and out of engagement with a film of ink formed by a flexible doctor blade urged into engagement with the hard surface of an ink fountain roller by a multiplicity of ink keys.
The ink film formed on the ink fountain roller has been too thick and too irregular for application directly to a printing plate for quality printing. These inkers which include a multiplicity of rollers are intended to reduce the thickness of the ink film and to deliver a film of uniform thickness to the printing plate. However, since the ink film on each form roller is not totally replenished on each revolution of the form rollers; image ghosting and ink accumulation and starvation is not eliminated.
The multiple roller inkers require complex drive trains and are relatively expensive to purchase initially and to maintain thereafter.
In an attempt to eliminate both the expense and the disadvantages of multiple roller inkers, many recent attempts have been made to develop inkers wherein a fresh film of ink is metered onto a form roller which is urged into pressure relation with a printing plate to eliminate the train of rollers, to eliminate image ghosting, and to eliminate ink accumulation and starvation.
U.S. Pat. No. 3,283,712 describes an inking system devised to overcome ghosting. The system comprises two rollers of substantially equal diameter urged together in pressure indented relation to form a nip, surfaces of the rollers adjacent the nip moving in opposite directions. One of the rollers was cleaned by a pair of doctor blades and the rollers were urged together such that the local pressure at any point selected along the contact generatrix or nip was greater than a "critical pressure threshold," such that, theorettically, one of the rollers carried a film of ink of constant thickness throughout the length of the roller to be applied directly to a printing plate without being contacted by equalizer rollers.
Ideally, a stationary metering unit requiring no drive in addition to that required for rotating a single form roller would appear to be a solution to the problems presented by previous inkers. Attempts have been made to employ doctor blades as ink metering units, but these attempts have universally met with failure. Doctor blades are successfully used as ink wiping units in inkers having a train of rollers for distributing and smoothing the ink, but such blades have not proven suitable for use as the sole ink metering unit for a resiliently surfaced form roller.
Printing ink is generally an oily viscous substance which is highly pigmented and formulated to be sticky or tacky so that the ink will properly adhere to image areas of the printing plate.
Ink generally employed for printing newspapers has a viscosity in a range of about 50 to 80 poise. Ink generally employed for letterpress printing and heat set inks employed for web offset printing have a viscosity in a range of about 150 to 200 poise. Inks employed in sheet fed lithographic offset printing presses is generally in a range of 250 to 300 poise.
When the image area of the printing plate transfers ink directly to paper or to a blanket cylinder which in turn transfers ink to paper, small paper fibers, lint and fragments of coating material may adhere to the surface of the plate cylinder. The plate causes the foreign substance to be applied to the surface of the ink applicator roller. If the surface of the ink applicator roller is moved directly into the reservoir and then wiped or scraped by a conventional doctor blade, the foreign substance tends to collect at the edge of the doctor blade which results in formation of an irregular film of ink on the surface of the roller. For this reason, in addition to the erratic behavior of the surface of the resilient roller under dynamic conditions, no inking device has been devised heretofore which is capable of supporting a doctor blade for metering a uniform film of ink directly onto the surface of a resilient roller in rolling engagement with a printing plate.
U.S. Pat. No. 3,298,305 discloses an inking mechanism having a stationary, rigidly supported edge held in a position to significantly indent a resilient roller surface such that a film forming portion on the inking mechanism would form a thin uniform film of ink which was delivered through a slot in the inking mechanism and applied to the roller surface. The edge was described as being positively locked in position to prevent any lifting by the ink film on the roller so as not to detrimentally affect the hydrodynamic effect.
U.S. Pat. No. 4,007,682 discloses a method of inking a resilient surfaced form roller wherein an ultra-thin doctor blade is mounted at a reverse angle to the ink to split the ink and apply the ink to the roller in the desired thickness when relative motion is provided between the roller and the doctor blade. The doctor blade is described as being flexible, for example, a blade constructed of Swedish steel having a thickness of 0.008 inches in one example and a thickness of 0.015 inches in another example.
The disclosure of U.S. Pat. No. 4,007,682 states that when an ink of high viscosity is used and the rate of relative motion between the roller surface and the edge of the blade is high, a sharp blade will "float" along on the ink surface, but the lead edge of the doctor blade should be cylindrical having a radius of curvature equal to one-half the thickness of the blade when less vicous inks are used. The disclosure states that the velocity of the roller surface relative to the doctor blade is adjusted to interact with the ink viscosity, blade geometry and downward force on the blade to cause the ink to be carried into the nip between the blade and the roller surface whereby its viscous resistance to shear forces creates an upward pressure causing the doctor blade to "float" over the ink film it produces. The surface speed of the roller is varied for varying the thickness of the film of ink formed thereon. The disclosure states that rotation of the inking roller at 68 inches per second provides a uniform coating of ink 5 microns thick and that when the rotation speed of the inking roller is increased to 172 inches per second a layer of ink 12 microns thick is formed.
The invention described herein addresses the problem of forming a film of printing ink of uniform thickness on a resilient roller surface and moving the film of ink into engagement with the image area on a printing plate while eliminating trains of rollers in inking systems, eliminating the necessity for dissipating heat due to consumption of excessive power for metering a thin uniform ink film, eliminating problems attendant to collection of "hickeys", providing a metering member which does not detrimentally stress a resilient roller surface so as to impart vibration to the resilient roller surface, and providing a metering member which forms a uniform film, the thickness of which is independent of press speed.